I'm processing a Landsat scene dated in year 2000-2001, so Toms Earth Probe data is the way to go. But as stated by @Tobi in an answer below, TOMS Earth Probe data is actually an index, and unitless (not really AOD at 550nm which I want in the 6S algorithm).

This is nice! If only the data isn't so random.. The data is user-submitted, right?
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user8723Nov 28 '12 at 1:02

@Haziq, did you use NASA's Aeronet data along with i.atcorr?
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Nikos AlexandrisJan 24 '13 at 11:46

Not really. I picked this as answer because it is an easy (and free) way to get data. The problem is I can't use them because they didn't exactly match the time/location of my data-set. I've decided to abandon my search (for now) and opted for the simpler DOS1 method.. Sorry that I can't be much of a help..
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user8723Jan 24 '13 at 15:58

I see, that's one way to do it.. But I've decided to pursuit this route of atmospheric correction at other time and have opted (for now) the simplified DOS1 method. Thanks for the clarification of the TOMS Earth Probe data: it is indeed an index and unit-less - not quite like the AOD data required by the 6S algorithm.. But I have yet to find free AOD data readily usable for the period of Landsat I'm using (<2004). But with further reading here, the data could still be processed further to develop the model. Maybe.
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user8723Nov 27 '12 at 22:43

Hey, I was wondering where to get the MODIS AOD data (it's not available on their site).. I'll take a look at this site thoroughly later; so far the data I can see there is in the same form as the TOMS data - Aerosol Index (man, NASA websites sure are complicated to browse through - I got a headache just by looking at it).. Anyway, thank you so much @Tobi :)!
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user8723Nov 28 '12 at 1:15